This invention relates to a pipe construction and, more particularly, to a means for mechanically coupling flexible thermoplastic pipe together. The method and apparatus relates to technology frequently found on the mining, petroleum exploration building sprinkler systems and heavy industry fields. The method and apparatus according to this application solves, in an inexpensive and straightforward manner, problems which have existed in these and similar fields since the advent of thermoplastic pipe as a fluid conveyance medium.
The use of thermoplastic pipe has replaced metal pipe in many applications because of the substantial advantages inherent in plastic pipe products. Thermoplastic pipe is relatively resistant to degradation from environmental contaminants, is lightweight, easy to cut and form, and relatively flexible. However, some of these advantages also create disadvantages when it is necessary to join together lengths of thermoplastic pipe to form longer segments. By definition, thermoplalstic pipe is relatively sensitive to the application of heat. While in many respects this is an advantage, one disadvantage resulting from this property is the substantial amount of expansion and contraction which can take place along a relatively long length of thermoplastic pipe caused by environmental conditions, i.e., air temperature, direct exposure to sunlight and the temperature of the fluid being transferred through the pipe. Numerous expensive and complicated methods and products have been proposed for joining segments of thermoplastic pipe together. One of these prior art methods is thermal butting or socket fusing segments of the pipe together. However, this ordinarily requires semi-skilled workmen and a suitable environment with relatively clean and dry work conditions. Therefore, this method of coupling is not entirely satisfactory for use in many production environments.
Alternatively, rigid inserts made of metal or rigid plastic are used to mechanically couple sections of flexible thermoplastic pipe with an outside clamp-type connector. These inserts reduce the inside diameter of the pipe and very often are not compatible with the liquid, slurry or gas flowing through the pipe. Also, the inside diameter of the pipe often does not exactly match the diameter of the insert rendering the pipe unsatisfactory for use with inserts. Thermoplastic pipe diameters and wall thicknesses in the past few years have increased substantially as thermoplastic pipe has been adapted to more severe operating conditions. Using the insert method to connect larger diameter and thicker-walled thermoplastic pipe together has in many instances become impractical.
Another method is to roll-groove or cut-groove the ends of thermoplastic polyethylene sections for joining with a roll or cut-groove coupling. In some stable, non-severe environments, this is a viable alternative.
This involves the formation, either by molding or cutting, of an annular groove near the end of a plastic pipe to be joined. Two such formed pipes are joined (or very nearly joined) end to end and are then connected together by means of a clamp which includes an annular rim on opposite sides, one of which mates into the groove of one of the sections of pipe. The clamp is tightly secured around the pipe and the rims hold the pipe together against movement along the axis of the pipe by engagement with the pipe grooves. This type of clamp allows for some expansion and contraction, deflection and a certain amount of vibration and noise absorption. In addition, gaskets of various pipes can be used to fluidly seal the pipe joint.
Finally, other mechanical couplings exist which have internal sharp teeth which penetrate into the outside surface of the pipe wall. These mechanical couplings are used primarily for low pressure applications. Use of these couplings with high pressures at or near the maximum pipe pressure rating or with end-pull due to the creep of thermoplastic polyethylene expansion and contraction causes the teeth to gradually cut through the pipe. This is a particular problem since polyethylene "cold flows." This simply means that the amorphous molecular structure of the plastic moves in response to stress, even when the pipe itself appears to be in a completely solid state.